Seat belt restraint systems for automobiles often include a pretensioner that is structured to apply tension to the seat belt when an impact event such as an accident situation is detected. When the pretensioner is activated, the pretensioner eliminates any slack in the seat belt, and thus controls the physical space between the occupant and the seat belt. In this manner, the occupant of the seat is coupled with the seat belt as the occupant initially moves forward relative to the seat, thereby controllably restraining the occupant, reducing occupant excursion, and preventing undue loads when the occupant moves forwardly into the seat belt.
A retractor is another standard component of a seat belt restraint system that includes a spool receiving the webbing material of the seat belt. The spool is used to wind up and store the webbing. Generally, the spool is locked in place by a blocking element upon detection at an impact situation in order to restrain the occupant via the seat belt. Modern retractors have been designed having one or more force limiting elements that are structured to allow the spool to rotate and pay out the webbing material of the seat belt upon reaching predetermined force levels between the occupant and seat belt. In this manner, the restraint force imposed on the occupant can be limited in a controlled manner, thereby providing a certain load limitation characteristics. Recently, retractors have also been designed to include a pretensioner operatively coupled to the spool for taking up the seat belt and eliminating slack when a potential impact event is detected.
Despite these and other improvements to automobile restraint systems, the potential exists for load limitation disturbances, such as so-called locking dip, during transition of spool torque load between the pretensioner and the blocking element of the retractor. Locking dip refers to a limited amount of webbing extraction from the retractor in an impact situation before the retractor locking mechanism is fully engaged. Locking dip is undesirable since it permits forward movement of the occupant without effecting energy absorbing forces. Accordingly, there exists a need to provide a retractor having a pretensioner that minimizes or eliminates such load limitation disturbances.